I am searching for a way to "neatly" achieve the following result, as I am currently using titlesec, which causes a "warning", as it is not optimised for KOMA and I am using scrreprt for my thesis.
(Total LATEX noob)
So what I was trying to achieve was the format on the right:
1 | Chaptername

The chapters should not have any space above them (excuse my bad english), that's what I was using the titlespacing for. (It should be around 1,5 spacing, but I didn't know how to get that dynamic spacing, therefore shabbily "converted" it into pt)
Also I'd like to stick with the style of the chapters including the vertical bar |

Welcome to TeX.SX! You might want to have a look at \sectionlinesformat and friends, because they are the KOMA way of approaching this (the German version of the manual should be a good starter for you). But please make a minimal compilable code example (MWE) to help us help you.
– TeXnicianAug 13 '18 at 12:38

Note however that the difference in the two versions is only about 5.36nm and therefore not visible (and lost during rasterisation for print). The below code should works for older versions of KOMA, too, so if backwards compatibility does bother you, you should use the code below.

Since KOMA-Script 3.26 you can also use beforeskip=0pt,afterindent=false instead of beforeskip=-1sp. However the difference is not visible.
– SchweinebackeApr 3 at 7:47

@Schweinebacke thanks for mentioning it, I didn't know about it. Well the difference is roughly 5.36nm.
– SkillmonApr 3 at 8:45

Yes, the difference is invisible and moreover there are still a lot of KOMA-Script versions < 3.26 in usage. So your answer is not only absolutely correct, it is also the best/only solution if backward compatibility is needed.
– SchweinebackeApr 3 at 8:49